In Situ Recovery and In Situ Leaching of Selected Critical and Strategic Metals: A Critical Review of Geological Preconditions, Reactive Transport, Process Engineering, Environmental Risk, and Scale-Up (2015–2026)

In situ recovery (ISR), also termed in situ leaching (ISL), is increasingly considered a low-disturbance alternative to conventional mining. This critical review synthesizes an author-curated corpus of 80 records published from 2015 through 2026 and evaluates ISR/ISL for uranium, copper, ion-adsorption rare earth elements (REEs), gold, and emerging electrokinetic and bio-assisted systems. The central distinction is between extraction measured in laboratory or column tests and recoverability at wellfield scale. Viability requires a narrow overlap among orebody accessibility, hydraulic connectivity, mineral reactivity, lixiviant selectivity, hydraulic containment, surface-recovery efficiency, and post-leach restoration. Permeability is treated as a dynamic process variable because dissolution, precipitation, clay swelling, fines migration, and fracture evolution can continuously modify hydraulic sweep. The strongest industrial evidence remains associated with sandstone-hosted uranium and selected ion-adsorption REE systems, whereas hard-rock copper, gold, and several electrokinetic concepts remain more site-specific or demonstration-oriented. Reactive-transport models, tracers, machine learning, and digital wellfield control are improving design capability, but predictive uncertainty remains dominated by heterogeneity and incomplete representation of coupled geochemical and hydromechanical feedbacks. Environmental performance cannot be inferred from the absence of excavation alone; groundwater migration, residual reagents, mobilized contaminants, restoration difficulty, and long-term monitoring remain decisive. An evidence-gated scale-up framework is therefore proposed in which geological, metallurgical, hydraulic, environmental, and economic criteria must be satisfied together. ISR is best regarded not as a universal substitute for conventional mining but as a high-potential route for deposits that fall within a rigorously defined feasibility window.